I started to get this error after I updated @bryntum/gantt and @bryntum/gantt-react packages to a version greater then 5.3.0. On the version 5.3.0 I am not having this error.
I am able to select a date from the calendar picker, but I am not able to edit it manually. An error happens for all date formats that contain '.' character like DD.MM.YYY, D.M.YYYY, etc. (German date formats). However, I do not observe the same error for date formats that contain '/' character like DD/MM/YYYY (UK date format).
When I try to console log date when startDate format is set to 'DD.MM.YYYY', I always get undefined when manually changing the date:
Here is a screen record of the error: